Biologists have long recognized the role of Hawaii's extreme geographical isolation in shaping its flora and fauna. The Hawaiian Islands are about 4,000 km from the nearest continental source of species making it the most isolated archipelago on Earth. As a result of this extreme isolation, Hawaiian streams have relatively fewer species compared to their continental counterparts with many groups of organisms entirely missing. The Hawaiian aquatic insect fauna, for example, has no native stoneflies, mayflies, caddisflies, alderflies, or dobsonflies; however certain insect taxa such as the odonates (damselflies in particular) and certain diptera (flies) exhibit spectacular radiation of species into specialized stream environments.

The larger organisms (macrofauna) native to Hawaiian streams generally have marine connections. All of the native macrofauna exhibit amphidromy in that they all have marine larval phases. Five species of fishes, generally all referred to as 'o'opu by the early Hawaiians, are native to Hawaii. Natives, incidentally, include both endemic species, which are only found in a particular area, as well as indigenous species which naturally occur in a particular area but are found in other locality as well. All of Hawaii's stream fishes are now believed to be endemic except one, Awaous guamensis ('o'opu-nakea). Two gastropod snail species, Neritina granosa (hihiwai) and Neritina vespertina (hapawai), are endemic to Hawaii. The edible hihiwai inhabits mountainous stream reaches while the hapawai is found in deep estuaries of streams near the ocean. Two decapod crustaceans complete the list of amphidromous native Hawaiian stream macrofauna. Atyoida bisulcata ('opae-kalaole) is a swift-water inhabitant of mountainous stream reaches and a popular delicacy among island residents. The Hawaiian prawn (Macrobrachium grandimanus - 'opae-'oeha'a) is a relatively small endemic (less than 3 inches) which inhabits Hawaiian estuaries.

Hawaii's stream environments are poorly surveyed and to date there has not been a systematic inventory of extant or search for new aquatic species within- and among-islands. Little is known, for example, about Hawaii's native aquatic and semi-aquatic lymnaied snail fauna which may rival the extraordinary and well-documented radiation of Hawaii's terrestrial snails. The algae of Hawaiian streams, which support most food webs, are virtually unstudied taxonomically. It is critically important that such surveys and inventories be undertaken immediately to protect Hawaii's remaining freshwater biodiversity.

Threats to stream environments in Hawaii are many. Alien species introductions into streams are rapidly escalating on all islands disrupting natural stream functioning and increasing predation and competition on native species with few natural defenses. Water withdrawals from streams for potable water, agricultural and hydropower development has been identified as one of the primary factors degrading the biological quality of streams in the Hawaiian Islands. These disruptive forces are only expected to increase and intensify in the coming century in Hawaii unless conservation policies are strengthened and needed information and technologies are developed.

Aquatic Species Photos